The correct option is
C Sheep
Nouns are naming words. For example: 'Ram is a boy.' Here, both 'Ram' and 'boy' are nouns. 'Ram' is a proper noun and 'boy' is a common noun. There are different types of nouns; i.e.; proper nouns, common nouns, material nouns, abstract nouns, concrete nouns, countable nouns, uncountable nouns, compound nouns and collective nouns.
NOTE: The given options are all collective nouns.
Option A - A herd of cows. Hence, option A is incorrect.
Option B - A herd or memory of elephants. Hence, option B is incorrect.
Option D - A herd of horse. Hence, option D is incorrect.
Option C - A flock of sheep. Hence, option C is the correct answer.
'A flock of birds of a sheep.'